Battle of Lake Trasimeno

Gaius Flaminius.
Gaius was elected consul together withCn. Servilius Geminus. Gaius was a homo novus one of the new rich,
but from plebeian origin. Now his appointment had to be accepted by the gods or more correctly the augurs.
He feared the augurs who took the auspices to find out the will of the gods. When he was elected consul for the first time they had tried to stop him and they would certainly try this again.
He decided not to wait and left Rome immediately to join his army that was camped near Ariminium.
The senate was furious and sent
messenger to call him back. He refused. Even a letter from the senate
that ordered Gaius Flaminius to return made no difference. Polybius(3.81) depicts the man as an incompetent fool, but this does no justice to the man . He had been a successful consul before (defeated the Gaul), as well as a master of the horse and as a censor he had the via flaminia (=Faminius' road) constructed. In Ariminium he took over the army from Ti. Sempronius Longus, the former consul. From here he moved to Arretium. His colleague raised a new army in Rome and went to Ariminum.

Hannibal
Hannibal was marching from his camp near the Trebia
towards Rome. The Romans figured he would either take about the same route Trebonius had taken or keep the Apennines on his left. Hannibal went right through the mountains. While doing so he had to cross some marshes. This caused a lot of trouble and illnesses. It cost him the sight of one of his eyes. He passed Arretium where he saw Flaminius' army, but didn't stop to give battle. He moved on to Cortona, while plundering, looting and destroying, thus leaving behind an easy to follow trail. Then he slipped along the lake Trasimeno behind the hills on the left of the picture above. The picture is taken from Cortona.

If we strip all the negative emotion about Flaminius, we can say Flaminius did exactly what he had to do:

He sent a messenger to Geminus

He tried to keep in contact with the enemy

Reconnaissance
The ancient authors suggest he should have stayed where he was and waited for Geminus to show up with his army. Had he done that he would had have to guess where Hannibal had gone. No, following Hannibal was the right thing to do. Just the way he did it was stupid. Goldsworthy in his book "The fall of Carthage" supposes reconnaissance wasn't a habit of the Romans. True, every time they didn't they walked right into a trap. In the 2d Samnite war even twice, but on numerous other occasions they sent out scouts or even the dictator himself reconnoitred.(See Livius (3.8; 3.28; 3.40; 4.39;4.46; 9.23 and 9.36) to name a few. What Gaius should have done was order a small group of scouts to follow Hannibal and report him about the Carthaginians whereabouts. But he didn't. Why? In his former consulship he was denied a triumph. That wasn't going to happen this time. Loosing the connection with Hannibal and Servilius Geminus getting away with the glory if the senate successfully had him, Flaminius, removed. Unthinkable. By the ancient authors he is accused of being eager to attack, but that is far from sure. He knew Hannibal had 40,000 troops and he just 25,000. Possibly he meant to follow Hannibal until the Roman armies could be combined.

The ambush

While the exact place of the ambush is unclear. The level of the water of the lake has considerably changed over the centuries and two villages claim "their Hannibal", which means more tourists.
An expert, who is willing to find the right arguments, is easily found. To me this discussion is not really important. In the small museum in Tuoro where had been an explanation of the battle on their ground, now the second theory is presented as a possibility.
Still, I think the place of the trap cannot have been on the north east side of the lake! Why? The distance between Flaminius' camp and that place is about 18km. If they left the camp about 7, they would have reached that presumed trap at about noon and not early in the morning. The fog would have been lifted. Flaminius camp cannot have been more to the east simply because there is no room for a camp of 25.000 man + horses.

The plan
A good look at how Hannibal divided his troops can tell us much about his plan. He placed his strongest forces at the entrance and exit. Slingshots and men with javelins have hardly any attacking power. A slingshot is useless when other men are in a hand to hand fight. They had probably orders to keep the middle of the Roman column in place as long as possible, while the stronger forces hacked their way to the middle. Hannibal (instinctively?) used an important principle: Make sure you have the upper hand, if necessary local. He concentrated his power at the two ends if the column.
Possibly he let the Roman vanguard pass in order to overpower the Romans in the trap even more, which is one explanation why they could escape. Another explanation can be that the Romans did what they should:immediately attack towards the enemy. If the did that in the middle of the column they faced the weaker part of Hannibal's army and could escape there. But it is very unlikely the the vanguard would have defeated Hannibal's strongest forces.
BothLivis and Polybius and others after them tell us Hannibal took his full army to the place of the fake army camp and then had them return to the designed places during the night by a detour!! This is very improbable! Riding horses over unknow mountenous terrain is asking for trouble: horses breaking there legs in every possible hole. Hannibal was a soldiers general who would think of the wellbeing of his troops. If he could save them 25km march he would do so. More likely he had scouted the trap in advance. Then he made the plan and shared that with his lieutenants, who in their turn placed the men in their places when they when the army passed that place. This would be done in broad daylight and save the men and horses a day (and night) of march.

Servilius' 4000 horse
When Servilius got the message Hannibal had been seen near Arretium he immediately sent 4000 horse. The distance between Ariminium and Arretium as a crow flies is about 100 km. But due to the mountains the real distance is 200km. On horse back that would take at least 3 days if the messenger could change horses otherwise 4 at least. In forced marches a Roman army could do 25km in a day. So Flaminius would need 3 days to reach the shores of the lake. Probably Servilius got the message at the day Flaminius died. His 4000 horse arrived about 5 days after the battle and were defeated by one of Hannibal's lieutenants Maharbal